{"id":68580,"date":"2020-09-05T06:52:02","date_gmt":"2020-09-05T06:52:02","guid":{"rendered":"https:\/\/www.baeldung.com\/?p=86644"},"modified":"2020-09-05T06:52:02","modified_gmt":"2020-09-05T06:52:02","slug":"illegalmonitorstateexception-in-java","status":"publish","type":"post","link":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/2020\/09\/05\/illegalmonitorstateexception-in-java\/","title":{"rendered":"IllegalMonitorStateException in Java"},"content":{"rendered":"

<\/p>\n

1. Overview<\/h2>\n
<\/div>\n

In this short tutorial, we’ll learn about java.lang.IllegalMonitorStateException.\u00a0<\/em><\/p>\n

We’ll create a simple sender-receiver application that throws this exception. Then, we’ll discuss possible ways of preventing it. \u00a0Finally, we’ll show how to implement these sender and receiver classes correctly.<\/p>\n

2. When Is It Thrown?<\/h2>\n
<\/div>\n

The IllegalMonitorStateException<\/em> is related to multithreading programming in Java. If we have a monitor<\/a><\/em> we want to synchronize on, this exception is thrown to indicate that a thread tried to wait or to notify other threads waiting on that monitor, without owning it. In simpler words, we’ll get this exception if we call one of the wait()<\/em>, notify(),<\/em> or notifyAll()<\/em> methods of the Object<\/em> class outside of a synchronized<\/em><\/a> block.<\/strong><\/p>\n

Let’s now build an example that throws an IllegalMonitorStateException<\/em>. For this, we’ll use both wait()<\/em> and notifyAll()<\/em> methods to synchronize the data exchange between a sender and a receiver.<\/p>\n

Firstly, let’s look at the Data<\/em> class that holds the message we’re going to send:<\/p>\n

public class Data {\r\n    private String message;\r\n    public void send(String message) {\r\n        this.message = message;\r\n    }\r\n    public String receive() {\r\n        return message;\r\n    }\r\n}<\/code><\/pre>\n
Secondly, let’s create the sender class that throws an IllegalMonitorStateException <\/em>when invoked. <\/em>For this purpose, we’ll call the notifyAll() <\/em>method without wrapping it in a synchronized<\/em> block:<\/p>\n
class UnsynchronizedSender implements Runnable {\r\n    private static final Logger log = LoggerFactory.getLogger(UnsychronizedSender.class);\r\n    private final Data data;\r\n    public UnsynchronizedSender(Data data) {\r\n        this.data = data;\r\n    }\r\n    @Override\r\n    public void run() {\r\n        try {\r\n            Thread.sleep(1000);\r\n            data.send(\"test\");\r\n            data.notifyAll();\r\n        } catch (InterruptedException e) {\r\n            log.error(\"thread was interrupted\", e);\r\n            Thread.currentThread().interrupt();\r\n        }\r\n    }\r\n}<\/code><\/pre>\n
The receiver is also going to throw an IllegalMonitorStateException. <\/em>Similarly to the previous example, we’ll make a call to the wait()<\/em> method outside a synchronized<\/em> block:<\/p>\n
public class UnsynchronizedReceiver implements Runnable {\r\n    private static final Logger log = LoggerFactory.getLogger(UnsynchronizedReceiver.class);\r\n    private final Data data;\r\n    private String message;\r\n    public UnsynchronizedReceiver(Data data) {\r\n        this.data = data;\r\n    }\r\n    @Override\r\n    public void run() {\r\n        try {\r\n            data.wait();\r\n            this.message = data.receive();\r\n        } catch (InterruptedException e) {\r\n            log.error(\"thread was interrupted\", e);\r\n            Thread.currentThread().interrupt();\r\n        }\r\n    }\r\n    public String getMessage() {\r\n        return message;\r\n    }\r\n}<\/code><\/pre>\n
Finally, let’s instantiate both classes and send a message between them:<\/p>\n
public void sendData() {\r\n    Data data = new Data();\r\n    UnsynchronizedReceiver receiver = new UnsynchronizedReceiver(data);\r\n    Thread receiverThread = new Thread(receiver, \"receiver-thread\");\r\n    receiverThread.start();\r\n    UnsynchronizedSender sender = new UnsynchronizedSender(data);\r\n    Thread senderThread = new Thread(sender, \"sender-thread\");\r\n    senderThread.start();\r\n    senderThread.join(1000);\r\n    receiverThread.join(1000);\r\n}<\/code><\/pre>\n
When we try to run this piece of code, we’ll receive an IllegalMonitorStateException<\/em> from both UnsynchronizedReceiver<\/em> and UnsynchronizedSender<\/em> classes:<\/p>\n
[sender-thread] ERROR com.baeldung.exceptions.illegalmonitorstate.UnsynchronizedSender - illegal monitor state exception occurred\r\njava.lang.IllegalMonitorStateException: null\r\n\tat java.base\/java.lang.Object.notifyAll(Native Method)\r\n\tat com.baeldung.exceptions.illegalmonitorstate.UnsynchronizedSender.run(UnsynchronizedSender.java:15)\r\n\tat java.base\/java.lang.Thread.run(Thread.java:844)\r\n[receiver-thread] ERROR com.baeldung.exceptions.illegalmonitorstate.UnsynchronizedReceiver - illegal monitor state exception occurred\r\njava.lang.IllegalMonitorStateException: null\r\n\tat java.base\/java.lang.Object.wait(Native Method)\r\n\tat java.base\/java.lang.Object.wait(Object.java:328)\r\n\tat com.baeldung.exceptions.illegalmonitorstate.UnsynchronizedReceiver.run(UnsynchronizedReceiver.java:12)\r\n\tat java.base\/java.lang.Thread.run(Thread.java:844)\r\n<\/code><\/pre>\n
3. How to Fix It<\/h2>\n
<\/div>\n
To get rid of the IllegalMonitorStateException,\u00a0<\/em>we need to make every call to wait()<\/em>, notify(),<\/em>\u00a0and notifyAll() <\/em>methods within a synchronized<\/em> block.<\/strong> With this in mind, let’s see how the correct implementation of the Sender<\/em> class should look like:<\/p>\n
class SynchronizedSender implements Runnable {\r\n    private final Data data;\r\n    public SynchronizedSender(Data data) {\r\n        this.data = data;\r\n    }\r\n    @Override\r\n    public void run() {\r\n        synchronized (data) {\r\n            data.send(\"test\");\r\n            data.notifyAll();\r\n        }\r\n    }\r\n}<\/code><\/pre>\n
Note we’re using the synchronized<\/em> block on the same Data<\/em> instance we later call its notifyAll()<\/em> method.<\/p>\n
Let’s fix the Receiver<\/em> in the same way:<\/p>\n
class SynchronizedReceiver implements Runnable {\r\n    private static final Logger log = LoggerFactory.getLogger(SynchronizedReceiver.class);\r\n    private final Data data;\r\n    private String message;\r\n    public SynchronizedReceiver(Data data) {\r\n        this.data = data;\r\n    }\r\n    @Override\r\n    public void run() {\r\n        synchronized (data) {\r\n            try {\r\n                data.wait();\r\n                this.message = data.receive();\r\n            } catch (InterruptedException e) {\r\n                log.error(\"thread was interrupted\", e);\r\n                Thread.currentThread().interrupt();\r\n            }\r\n        }\r\n    }\r\n    public String getMessage() {\r\n        return message;\r\n    }\r\n}<\/code><\/pre>\n
If we again create both classes and try to send the same message between them, everything works well, and no exception is thrown.<\/p>\n
4. Conclusion<\/h2>\n
<\/div>\n
In this article, we learned what causes IllegalMonitorStateException <\/em>and how to prevent it.<\/p>\n
As always, the code is available over on GitHub<\/a>.<\/p>\n
The post IllegalMonitorStateException in Java<\/a> first appeared on Baeldung<\/a>.<\/p>\n
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<\/p>","protected":false},"excerpt":{"rendered":"
If you want to learn why your code keeps throwing this exception, this article is for you.<\/p>\n
The post IllegalMonitorStateException in Java<\/a> first appeared on Baeldung<\/a>.<\/p>\n
<\/a> <\/a> <\/a> <\/a> <\/a> <\/a> <\/a> <\/a> <\/div>\n<\/div>","protected":false},"author":214,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"slim_seo":{"title":"IllegalMonitorStateException in Java - ITTeacherITFreelance.hk","description":"If you want to learn why your code keeps throwing this exception, this article is for you. The post IllegalMonitorStateException in Java first appeared on Baeld"},"footnotes":""},"categories":[6],"tags":[5059],"_links":{"self":[{"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/posts\/68580"}],"collection":[{"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/users\/214"}],"replies":[{"embeddable":true,"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/comments?post=68580"}],"version-history":[{"count":8,"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/posts\/68580\/revisions"}],"predecessor-version":[{"id":72478,"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/posts\/68580\/revisions\/72478"}],"wp:attachment":[{"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/media?parent=68580"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/categories?post=68580"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/tags?post=68580"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}